Height gauges, coordinate measuring machines, profile measuring machines and the like have been known as measuring machines that measure shape and dimension of an object to be measured (sometimes simply referred to as an “object” hereinafter). Among such measuring machines, contact-type measuring machines use a touch signal probe that is configured to detect a contact with an object to be measured. There are various types of known touch signal probes, one of which is an ultrasonic touch trigger probe (see, for instance, Patent Literature 1: JP-A-06-221806). The ultrasonic touch trigger probe applies vibrations to a stylus using a piezoelectric element and monitors amplitudes and/or frequency change of the vibrations to detect a contact with an object.
In detecting a contact using the piezoelectric element, the piezoelectric element is resonantly excited by a positive feedback control and generates a detection output signal in a form of a sinusoidal signal, which is outputted from a detection electrode of the piezoelectric element. The amplitude and frequency of the sinusoidal signal varies when a contact piece touches an object to be measured. For instance, in order to detect a contact based on amplitude information, an amplitude change detector is used in a first typical contact detector circuit as shown in FIG. 6. The amplitude change detector detects and extracts only the amplitude information from the obtained sinusoidal signal to obtain a DC sensor signal S, and generates a trigger signal TG at a point where a DC level of the DC sensor signal S is rapidly decreased due to the contact. The amplitude change detector generates the trigger signal TG based on a comparison between the DC sensor signal S and a constant reference electric potential VREF.
However, when the reference electric potential VREF of a constant value is used for contact detection of a probe, the probe may erroneously detect a contact with an object although the probe actually does not touch the object. The erroneous detection occurs for the following reason. In most cases, the DC sensor signal S has a DC offset component, a low-frequency state change component showing a slow amplitude change due to beat etc. and a high-frequency noise component that are superimposed with each other. Accordingly, depending on a setting of the reference electric potential VREF, the low-frequency state change component and the high-frequency noise components are overlapped with the reference electric potential VREF at a point other than the point at which the DC level of the DC sensor signal S rapidly varies as shown in FIG. 6. At such a point, the amplitude change detector occasionally erroneously detects that the probe touches the object and outputs a detection signal in error. The erroneous detection accounts for a large part of errors in the physical amount to be detected.
In order to avoid such an erroneous detection, as shown in FIG. 7, the DC offset component S0 and the low-frequency state change component S1 are extracted from the DC sensor signal S using a low-pass filter, and the amplitude of the DC sensor signal (S0+S1) including the low-frequency state change component S1 is converted to obtain a reference signal (signal K(S0+S1)) (see, for instance, Patent Literature 2: JP-A-10-111143).
However, as shown in FIG. 7, when, for instance, the touch signal probe is kept in contact with the object, a second typical contact detector circuit shown in FIG. 7 sometimes detects that the probe is out of contact with the object and issues an erroneous signal even while the probe is still in contact with the object.
A technique for avoiding the above-described erroneous contact detection and erroneous non-contact detection has been proposed (see, for instance, Patent Literature 3, JP-A-2009-276238).
The technique disclosed in the Patent Literature 3 is based on the disclosure of the above-described Patent Literature 2.
As shown in FIG. 8, a third typical contact detection circuit 300 disclosed in the Patent Literature 3 includes a low-pass filter 301, a converter circuit 302, a selector circuit 303, a comparator circuit 304 and a converter circuit 305.
As shown in FIG. 9, the above-described third typical contact detection circuit 300 compares a DC sensor signal S and a reference signal R using the comparator circuit 304, and generates a trigger signal TG when the DC sensor signal S falls below the reference signal R.
When the trigger signal TG is not generated by the comparator circuit 304, the selector circuit 303 uses a signal R1=K(S0+S1) obtained by converting the amplitude of the DC sensor signal (S0+S1) as the reference signal R. The value of R1 varies in accordance with (S0+S1).
On the other hand, when the trigger signal TG is generated by the comparator circuit 304, the second converter circuit 305 calculates a signal R2=m·Rt1 with reference to a sampling value Rt1 of the reference signal R at a time t1 when the trigger signal TG is outputted. The selector circuit 303 uses the calculated signal R2 is used as the reference signal R. The value of the signal R2 is constant and unchanged.
When the signal S exceeds the signal R2=M=Rt1 (the reference signal) the trigger signal TG declines (time t2), the selector circuit 303 again uses the signal R1 as the reference signal R.
However, the technique disclosed in the Patent Literature 3 requires calculations of both of the signal R1=K(S0+S1) and the signal R2=m·Rt1 for obtaining the reference signal R, so that the arrangement becomes complicated.